Von Willebrand factor (vWf; Factor VIII-related antigen, R:Ag) has been found on and within normal platelets. This pool of platelet-vWf is intrinsic to the platelet and non-exchangeable with circulating plasma vWf. Any interaction of this platelet-vWf with plasma coagulation factors is merely speculative at this time as is the role it plays in platelet mediated primary hemostasis. This project seeks to quantitate the amount of platelet-vWf located on/within the platelet. Additionally, the studies will attempt to determine what role platelet-vWf plays in platelet activation, primary hemostasis, and plasma coagulation. By using this system to identify platelet-vWf on the surface of platelets, we hope to be able to identify circulating "activated" platelets and to better understand the nature of the processes by which platelets become activated.